In recent years, thermoplastic elastomers having excellent productivity are increasingly used as automobile parts, home electrical appliance parts, medical parts and general merchandise applications, which used to be mainly made of a vulcanized rubber. Examples of the thermoplastic elastomers include an olefin elastomer consisting of an ethylene-propylene copolymer and polypropylene, a polyurethane elastomer and soft polyvinyl chloride.
However, under the circumstances, the above-mentioned thermoplastic elastomers have disadvantages in respect of scratch resistance, flexibility, processability, economical efficiency and recycling efficiency.
That is, although the olefin elastomer is relatively inexpensive, and has excellent weatherability and heat resistance, the olefin elastomer has inferior flexibility and scratch resistance.
Although the polyurethane elastomer has excellent scratch resistance, the polyurethane elastomer has disadvantages that it has inferior flexibility and weatherability and is expensive.
Although the soft polyvinyl chloride is relatively inexpensive and has excellent weatherability and scratch resistance, the soft polyvinyl chloride has disadvantages that it has inferior low-temperature flexibility and recycling efficiency.
Various techniques are proposed for an elastomer composition using a hydrogenated derivative of a copolymer of a vinyl aromatic compound—a conjugated diene compound (hereafter, sometimes abbreviated as a hydrogenated block copolymer). For example, Patent Documents 1 to 3 disclose a composition obtained by blending a rubber softener and an olefin resin with a hydrogenated block copolymer (for example, see the following Patent Documents 1 to 3).
An airbag system includes a collision sensing device for detecting the collision of a high-speed movable body, and an airbag device in principle. The latter airbag device includes an airbag, a gas generator for generating a gas inflating the airbag, a storage cover for storing the airbag and the gas generator, and a bracket (retainer) for mounting the storage cover on the gas generator.
The airbag device is mounted on the front side of an occupant moving at high speed. When the collision sensing device is operated by collision, a gas is momentarily generated from the gas generator. The airbag folded and stored in a space surrounded with the gas generator, the storage cover and the bracket is filled with the gas. The storage cover is deployed by the pressure of the gas with which the airbag is filled. The airbag is momentarily released and expanded to the front side of the occupant from an opening part obtained by the deployment to restrain the occupant to the seat side. As a result, the airbag prevents the occupant from being injured by collision with a steering device and a dashboard panel or the like.
Therefore, the storage cover of airbag devices must be certainly deployed without scattering fragments which may wound the occupant, and release the bag momentarily when a collision accident or the like occurs to operate the gas generator.
A storage cover of airbag devices made of an urethane resin containing reinforcing nets which is mainly made of nylon is proposed (for example, see Patent Documents 4, 5).
A storage cover is proposed, which has a structure where a surface layer made of a soft material having a JIS K6301 A hardness of 30 to 70 and a core layer made of a hard resin having elasticity are integrally formed and a cleavage slit is formed in the core layer (for example, see Patent Document 6).
A storage cover of airbag devices is proposed, which is made of a thermoplastic elastomer composition with a JIS K6301 A hardness of 60 to 85 containing a hydrogenated styrene-conjugated diene block copolymer as a main component (for example, see Patent Document 7).
A thermoplastic elastomer composition is proposed, which contains two kinds of hydrogenated styrene-conjugated diene block copolymers having a comparatively low-molecular weight as a main component (for example, see Patent Document 8). A thermoplastic elastomer composition is proposed, which contains a hydrogenated styrene-conjugated diene block copolymer having a high-molecular weight and a hydrogenated styrene-conjugated diene block copolymer having a comparatively low-molecular weight as a main component and it is suggested that this composition can be used for a storage cover of airbag devices (for example, see Patent Document 9).    Patent Document 1: Japanese Patent Application Laid-Open No. 50-14742    Patent Document 2: Japanese Patent Application Laid-Open No. 52-65551    Patent Document 3: Japanese Patent Application Laid-Open No. 58-206644    Patent Document 4: Japanese Patent Application Laid-Open No. 50-127336    Patent Document 5: Japanese Patent Application Laid-Open No. 55-110643    Patent Document 6: Japanese Patent Application Laid-Open No. 1-202550    Patent Document 7: Japanese Patent Application Laid-Open No. 5-38996    Patent Document 8: National Publication of International Patent Application No. 9-530779    Patent Document 9: Japanese Patent Application Laid-Open No. 11-209559